Spark plug construction



SPARK PLUG CONSTRUCTION Filed Dec. 5, 1958 s i f' .f a' ALfxAA/pfe L. R056,

' chamber to .much higher.y temperatures than minimum temperatures ,to be kept clean, but 45 formerly hadA been the case. Under these cirmust Vnot exceed certain temperatures to ,pre-

Patented May 28,; l I

UNITED STATES y PATENT OFFICE SPARK PLUG coNvsfrnUc'rloN Y Alexander L. nnbh,- rasedena, oeuf., assigner', by direct and mesne assignments, to R & R. Mfg. go., Los Angeles, Calif., a corporation of Caliorma Y Application December' 5, 1938. Serial No. 243,994

1o claims. (on 12s-169) The present invention relates generally to. vwhich they are designed, lreach or stay at a spark plugs for internal combustion engines and sufficiently high temperature to pre-ignite the the like, and more especially to improvements in fuel charge. spark plugs designed to widen the satisfactory Plugs of the coldf type are suited only to operating range of a plug of given design and at relatively hot engines and fail to operate in 5 the same time improve its heavy duty eiliciency. Cold engines, in Which COmPalatVelY W In earlier'types of internalcombustion engines mean operating temperatures prevail, because the conditions encounteredl were more uniform the plugs are fouled easily by carbon deposits and not so severe as the conditions-found at the on the insulating -portion of the plug. Carbon present day. The mean operating temperatures deposits either may cause a short circuit that l10 were relatively low, measured by present standdestroys the effectiveness of the spark across ards, and so .only a relativelynarrow range of the spark gap, or else increase the resistance operating conditions was encountered. Under across the gap to the point where no spark at those conditions, spark plugs were practically all is generated. This condition of fouling has universal in that one type of plug would not only been solved by providing a plug of a so-called l5 be suitable for most, if not all, engines-but one "hOt type, .meaning that the Plug retains a type was suitablefor the fentire range of condigreater amount of heat between explosions and tions over which any one engine operated. The is maintained at a sufliciently high mean temdesign of lspark plugs was much simplified', the perature to burn ol carbon deposits and keep chief object being to make differentsizes of plugs the sparkgapV clean. 20 without changing the basic style or type. The type of plug, i. e., whether hot or cold,

But with time, internal combustion engines ,l is determined bythe mass ofthe electrode and Y have changed very much in use, style, output, 'surrounding insulating medium, usually porceand the like, and the operating conditions have lain, -exposed to and heated: by the gases in the changed correspondingly. High compression racombustion chamber. If the plug is of the hot tios, increased horsepower, higher speeds, new type, the massl of insulation and electrode is fuels, have all contributed to raising greatly the relatively large so that it cools slowly between mean operating temperatures and imposing explosions and retains sucient heat to keep much more severe operating conditions upon from fouling. The length of the exposed porcespark plugs. These new conditions have been lain or other insulation is reduced to decrease v met by changes in spark plug design which have the mass heated when a vcold plug is desired. been, at best, compromise solutions of the new The smaller massof electrode and insulation problems, and leave much to be desired in spark cools more rapidly and the spark plug operates plug performance. These short-comings may be at a mean temperature lower with respect to largely grouped under two headings: inability of the combustionA chamber mean temperature. spark plugs to perform satisfactorily over more 4In practice, it is necessary to make changes in than a relatively narrow range of operating conthe 'critical masses by small increments in order ditions, and inability to stand up'under heavy to provide the small gradations between extreme duty conditions. j a values, to adapt plugs to various motors, result- Many internal combustion engines of recent ing in many styles with attendant increase. in 40 design are termed, "hot since they maintain a costsand inconvenience to manufacturer, dealer, comparatively high mean temperature inthe and user.

combustion chamber, thus heatng the cylinder The requirements for hot and "cold plugs. walls and parts in and around the combustion are conflicting since a plug must reach certainV cumstances, and when the parts become insuivent pre-ignition of the fuel charge and decie'ntly cooled between explosions, it is easily struction of the plug. In actual Practice, spark possible for parts of the spark plug to remain plugs are designed to operate under a given set above the ignition point of the fuel vapor and of conditions, and do so with satisfaction only 50 cause pre-ignition of the charge. 'I'his is obvlas long as the deviation from the given condiously undesirable. In order to eliminate pretions is small becausethe fluctuations in tem- Vignition diiiiculties, there have Jbeen designed perature of the plug above and'below its mean types of plugs known to the trade as cold'plugs, temperature are not very great. The result is meaning that they do not, under conditions for that at the present time approximately twenty different types of plugs ranging from the coldest to the hottest are necessary to cover the full range of operating conditions found in present day engines; and even then the plugs are not sufficiently flexible or wide in their coverage to perform with high efficiency over the full range of many motors. Indeed, in some motors such as high compression, high speed racing motors, it is now necessary to use more than one set of plugs. A hot plug is used in the engine while the engine is being warmed up; then the plugs are removed and replaced by a cold plug that is able to perform at the high mean temperatures of maximum output. In other words, the range of conditions found in a single engine is so great that no single plug of conventional design will operate, even poorly, over the entire range of the engine. Plugs suitable for starting are destroyed and produce pre-ignition at high engine output, while at low output or when starting, the conventional cold" plug refuses to fire and does not function at all. 'Ihis example shows the measures necessary to overcome the limitations inherent in spark plugs of conventional design, and indicates that the flexibility of engines, such as are used in automobiles, is limited in many cases by spark plug deficiencies rather than by engine design.

It is,' of course, well known that heavy duty spark plugs used in racing and airplane engines have a relatively short life under thesesevere conditions. Porcelain is the most universally used insulating material, and the sustained high temperatures causa the glaze on the surface to crack and pit, facilitating formation of carbon deposits. Also, the porcelain is so thick that it cracks from internal strains -resulting from the high temperatures. 'I'hese occurrences, together with pitting of the electrodes, produce relatively rapid deterioration of the spark plugs at contin- `ued high temperature.

The above considerations show why known types of plugs have but narrow ranges of operation, these ranges being in most instances far less than the full range of engine operatingconditions. Consequently, both design and selection of a conventional plug for an engine is at best a balancing of the Various demands, a compromise between conflicting requirements rather than a satisfaction of the various requirements.

It is therefore a general object of my invention to provide an improved type of spark plug which satises the conflicting conditions imposed upon plugs adapted to operate in modern types of internal combustion engines and which has a greatly increased operating range so that one type of plug not only operates efficiently throughout the entire range of conditions encountered in any one engine, but also is substantially universalin its application to all types of engines.

Likewise, it is an object of my invention to change the character of the masses or parts of a spark plug that become heated during the explosion cycle in order to remove the present inherent limitations imposed upon spark plugs by conventional constructions.

Another object is to provide a spark plug having greatly increased life and efficiency.

These objects have been attained according to my invention, in a spark plug having a body and an electrode assembly located centrally within the body, by providing a layer of electrical insulating material interposed between the body and the electrode assembly, in order to insulate the body and electrode from each other, and an annularskirt of laminated dielectric material associated with the insulating layer and surrounding the lower end of the electrode assembly. The skirt is spaced from both the body and the electrode. The insulating layer between the body and the electrode is preferably of laminated dielectric material, preferably mica, although other suitable materialmay be used. In a preferred embodiment of my invention, this layer is in engagement for a portion of its length with the body and electrode assembly and extends beyond or outside the body to form the annular skirt which surrounds the lower portion of the central electrode and is annularly spaced therefrom. Said annular skirt is likewise spaced from the spark plug body and extends into the combustion chamber of the engine. Although in a preferred form the skirt andinsulating layer are formed integrally with each other, they may be separate members. The skirt is of suiliciently small mass that, although it becomes heated at each explosion within the combustion chamber, it cools rapidly during the expansion stroke to a temperature below that which will pre-ignite the incoming fuel charge, and consequently the skirt may be said to heat and cool with substantially the same cycle as the gases within the combustion chamber. The skirt may, if desired, be reinforced by'a thin metal shell around it, the mass of the shell being sufciently small that the combined masses of the shell and dielectric skirt still follow the thermal cycle of the gases within the cylinder.

How the above objectsand advantages of my invention, as well as others not specifically mentioned, are attained will be more readily lunderstood by reference to the following description and the annexed drawing, in which:

Fig. 1 is a vertical median section through a preferred form of spark plug;

Fig. 2 is a view, partly in elevation and partly in vertical section, through a modified form of spark plug;

Fig. 3 is an enlarged vertical section through the lower end of the electrode assembly and skirt of dielectric material; and

Fig. 4 is a vertical median section through a variational form of plug.

'I'he spark plug illustrated in Fig. 1 comprises a body portion I0 having a threaded barrel I I at its lower end by which the plug is screwed into a suitable opening in an internal combustion engine, a fragmentary portion of which is indicated at I2. The central electrode assembly is indicated generally at I5 and extends longitudinally or axially of body I0. This electrode assembly comprises a taper plug I6 carrying wire electrode I'I, which is the spark-forming portion of the assembly, and a hollow metallic tube I8 which is preferably made of stainless steel or like material. Surrounding electrode I5 and interposed between it and body l0, is a layer of laminated dielectric material 20, which is preferably made of mica although any other suitable material having similar properties may be used. The electrode and surrounding dielectric material 20 are in part within an internal cavity of body I0, the cavity comprising the upper bore 24 and a slightly tapered lower bore 2|. In this form of the lnvention, dielectric 20 extends beyond bore 2| and outside body I 0. Within bore 2l is a metal shell 22 which also extends downwardly below or outside the spark plug body, the shell being made of stainless steel or other similar material.

In assembling the parts of the spark plug, shell 22 is placed in bore 2|, the wallsof which converge upwardly, and the shell is swaged to t the bore and the rounded shoulder at top end of bore 2|. Then layer20 of laminated dielectric is inserted upwardly into shell 22. Next, tube) I8 is placed inside the insulating means 20 and the lower end expanded by swaging to hold the insulation tightly against shell 22. Taper plug I6 is then driven home to firmly hold the electrode assembly and shell 22 in place. The bottom. edge of tube I8 is preferably spun over the shoulder of plug I6, as in Fig. 3, to lock the plug in place.

The upper bore 24 of the internal spark plug cav-- ity is enlarged and lined at 25 with dielectric material, which also is preferably mica. 'Ihe annular space between the upper end of dielectric layer 20 and insulating lining 25 may be filled with a suitable insulating material, such as asbestos, as indicated at 26.

'I'he central electrode assembly is connected to the ignition system of the engine by an insulation-covered conductor 28 provided with a spring terminal Iplug 29 that is adapted to f'lt snugly Within the upper end of tube |8.` The top end of tube I8 is preferably flared outwardly as i1- lustrated to facilitate insertion of the conductor terminal. Some means, such as cap 30, is preferably provided on conductor 28 to close the upper .end of Athe spark plug bore 24 in order to exclude dirt, moisture, and other substances which would interfere with proper operation of the spark plug'.

In the construction shown in Fig. 1, a second or outer electrode 32 is mounted on the lower end of shell 22, the electrode being a small diameter wire bridging the shell and forming in conjunction with electrode II a spark gap at 33 across which the spark for ignition of the fuel charge is formed., While the illustrated constructiorrof electrodes I1 and 32 is preferred, for reasons which will be brought out later, it will be understood that the invention is not limited to the particular size. shape, or arrangement of electrodes illustrated, byt that other conventional electrode constructions may be used if desired.

Barrel is counterbored at 34 to a larger diameter than bore 2| in the spark plug body. Dielectric layer 20 extends downwardly below the limits of its engagement with the body and into the counterbore, the portion of the dielectric outside the body forming an annular skirt portion 20a which surrounds the lower end of the electrode assembly but is annularly spaced therefrom. Shell 22 also extends below body I0 and surrounds a portion of skirt 20a. It will be understood that the counterbore within barrel II is open to and forms, in effect, a portion of the combustion chamber of the engine, and consequently skirt 20a and the lower portion of shell 22 around the skirt may be considered as extending vinto the combustion chamber. That portion of shell 22 which is in bore 2| and actually contacts body I0, referred to as the upper portion, may be considered to be a part of the body. It is convenient to hold shell 22 in place by placing it between body I0 and dielectric 20, but other. means for fastening it in place may be employed. Since the portion of dielectric layer 20 within bore 2| is held in place between the body yand plug I6, it is considered to be in engagement with the body I0, although it is not in direct contact with thev body.

Fig. 2 illustrates a slightly modified form of spark plug which is the same as the form of Fig.

1 except that the electrode assembly has been projected farther into the combustion chamber, as is desirable in some types of engines.- 'I'his has been accomplished by eliminating counterbore 34 and moving the end of bore 2| down to the bottom of barrel Il 'I'he effect is to move the whole assembly I5 down with respect to body I0 and shorten the upper end of the body. The plug still has an internal cavity tcomprising bores 2| and 24, and the engagement of the dielectric 2U with the body II) terminates at the lower end of bore, and skirt 20a and shell 22 project below barrel II.

As above mentioned, insulating layer 20 and skirt 28a are both preferably made of a plurality of laminae of mica, and consequently may be combined or associated together in a single unit. To form this combined layer and skirt of laminated materiaL'sheetsof mica may be wrapped around tube I8 or an arbor a number of times to build upthe desired thickness of this insulating layer, the skirt thickness then being the same. The minimum thickness of the layer, indicated as the dimension t in Fig'. 3, is in general determined by the amount of insulation required between shell 22 and electrode tube I8. This minimum thickness is found in actual experience to be on the order of .015 inch, although a somewhat greater minimum is .preferable in practice from the stand-point 'of insuring an uninterrupted operation over a long time. The maximum thickness permissible is determined by the skirt and Will depend upon the type of engine in -'which the plug is to be used, but it Will be found mica sheets may be obtained in a thickness of approximately .001 inch, the insulating layer 20 will usually be composed of approximately 20 to 25 turns of this thickness; and the number of laminae in the layer decreases, of course, as the thickness of the wrapped sheet increases. The skirt 20a and layer 20 may not be of the same thickness, if desired, although the construction shown is preferred,

Shell 22 is preferably made lof sheet stainless steel having a thickness t of approximately .010

inch, or preferably less. The length of the shell is not critical, and it may extend below the skirt,

but it is preferred that it end above the bottom of the skirt by a distance, indicated at e in Fig. 3, of about one-eighth inch. The distance e may be greater in a short skirt because of the requirement of a minimum length f mica surface, dispussed below. The thickness of this shell may be varied somewhat within limits, according to principles which will be discussed, although if it isv less than .005 inch the shell affords very little reinforcement.

The length l of skirt 20a., that is, the distance that the dielectric material extends below or outside body 0, may likewise be varied. It has been found preferable in practice tomake the dimension I approximately three-eighths inch. although plugs have been observed to work satisfactorily with variations in this dimension ranging from approximately one-eighth inchto approximately lflve-eighths inch. The length of skirt is subject to the requirement that there should be at least one-fourth inch measured along the mica surface separating the central electrode from the body', and the measurement in this case is made from the bottom of shell 22 around the end of skirt a to the end of tube I8. If this is less than one-fourth inch, electricity. leaks over the mica surface to the body.

In general, it-may be said that the skirt thickthe skirt is apt to be too thin or too long; and-if too long it may melt or be broken away until it is shortened enough that'lt comes within the range. If the ratio is less than 4, the skirt may be too short to prevent electric leakage from the central electrode, or the thickness may be so much that the mass is too large to secure maximum advantages of the invention. When a shell 22 is yused, its mass must also be allowed for in designing the skirt. There T equals t-l-t, the ratio l/T is preferably between 3 and 25.

It is preferred to make insulating means 20 and skirt 20a integral with each other because it is easy to associate properly the two elements. It is, however, possible to make the two elements as separate members of different materials, and in this case the skirt is still associated with and contiguous to the insulating means which insulates the body from the central electrode assembly. An embodiment of this latter type is shown in Fig. 4 in which the central electrode assembly is a single formed electrode 40 extending longitudinally of the plug body 4|` and insulated therefrom by suitable insulating means 42, shown as a molded porcelain core or the like. The core 42 and electrode 40 are placed within an internal body cavity, the lower portion 43 of which is a tapered bore. Between the tapered f walls of bore 43and aconical portion of core 42,

is held a skirt 45 of laminated mica or other like material. Tapering these parts holds the skirt firmly when the insulating core 42 is drawn downwardly within the body cavity by turning down nut 46 which engages body 4I with a screw thread. Other suitable means of holdingA` the skirt in place may be employed. 'Ihe lower end 40a.of the central electrode is the sparking portion and forms a spark across an air gap to electrode 41 on body 4i.

vAs before, the skirt 45 extends beyond or o utside body 4I into that portion of the combustion chamber formed within the counterbore 34 of barrel 4Ia. Skirt 45 is in direct contact with both insulating means 42 and body 4l at'its upper end for a suiiicient length to hold it in place, but is entirely separate from the insulat ing means. Skirt 45 is in all essentials the same as skirt 20a as the length, thickness, material, and laminated construction of skirt 45 are all subject to the same considerations as are set out above in connection with Figs. 1 Vto 3. Shell 22 has been omitted.

The advantages of this novel spark plug construction are derived primarily from the use of a skirt which, together with the surrounding shell if one is used, has a relatively small mass, and also is of laminated nature. These advantages will be more readily 4understood if the operation of a spark plug having/a skirt composed of only a single mica sheet (about .001 inch)' is first considered. Tests of a spark plug of this aaoaoav nature disclose that the mica skirt melts back to within a very short distance of the spark plug body, indicating that the skirt of this small mass has reached temperatures of approximately 3000 F. during or shortly after the explosion in the combustionv chamber. Yet this skirt cools sufficiently that no detonation or pre-ignition of incoming fuel charge occurs. The results indicate that, although the skirt has such small mass that it becomes heated far above the ignition point of the fuel at each explosion within the cylinder, it cools below the ignition point as the gases are exhausted so that it is below the ignition point when the new fuel charge enters the cylinder. 'I'his rapid heating and cooling may be said to take place at the same rate or at the `same cycle as the heating and cooling of gases within the combustion chamber. To summarize the conditions, it has'been found that a skirt composed of a single sheet of mica is of such small mass that it closely follows the thermal cycle within the combustion chamber.

A plug having a skirt formed of only a single sheet of mica as mentioned, functions perfectly for awhile but has a relatively short life, and this life may be extended by increasing the number lof sheets of mica in the skirt. While the rate of heating and cooling is naturally somewhat modified by adding to the number of mica laminations, yet it has been found that a laminated skirt, even though composed of twenty or more laminae, behaves much more like a single sheet -than like a solid homogeneous body of similar dimensions. In other words, the laminated skirt of .the dimensions given still substantially follows the thermal cycle of the combustion chamber, and does so so much more closely than a non-laminated or single lamina skirt of the same dimensions as to produce greatly improved operation. As a consequence of this laminated construction, it has been found by experiment that a spark plug construction within the general limits set out above becomes sufficiently heated during each explosion stroke to keep the skirt 20a entirely clear of any carbon deposits that would interfere with the plugs efficiency, and at the same time the plug cools with such rapidity that there is no danger of pre-ignition. 'I'his condition has been observed to hold true under a very wide range of operating conditions. Consequently, a single plug of this type will cover substantially all, if not all, of the operating conditions found in engines of modern types. It will be realized, of course, that the dimensions and mass of the skirt and surrounding shell may be varied somewhat according as a plug may be designed especially to operate in relatively hot or cold ranges, or to increase the range of the plug toA meet new conditions not found at present in lengines. As mentioned above, any types of electrodes l1 and 32 may be used, but when the plugs are designed for service in hot engines, it is preferred to use platinum wire of about .010 to .O20 inch diameter. These line Wire electrodes follow the heat cycle in the cylinder and do not cause any pre-ignition.

Another advantage of the laminated construction of dielectric layer 20 is that although the wide operatingarange of one or a few thin sheets of mica is obtained, yet there is not the disadvantage that the mica becomes hot enough t0 melt. There is sufficient transfer of heat from the outer laminae to the inner ones that even at the extremely high temperatures encountered,

' and because of the laminated construction there are but little greater stresses set up in the skirt as a Whole. Consequently, the destruction orD deterioration of the insulation from heat stresses is substantially eliminated as compared with a solid homogeneous bodyof equal mass and dimensions.

As an example of the ability of this plug to perform under all conditions, plugs of this type have been found to perform perfectly at high output in an automobile engine using gasoline of 60 octane rating without pre-ignition when the motor had become sufliciently overheated that other plugs in the engine, of a conventional type in the best suited heat range, would miss ire or pre-ignite the charge. In the same engine it was found impossible to foul my improved plugs, by short-circuiting individual plugs and continuing to run the motor, or by coasting down yhill under compression with the ignition off. Under these conditions my novel plugs fl'red immediately and performed perfectly whereas the conventional plugs had to be removed and cleaned before they would fire at all.

The long life 'of my improved spark plug is illustrated by comparative tests run in stationary engines of the Hesselman type using Diesel fuel and run continuously over the entire output range. In engines of this type conventional spark plugs of the best heat range available have been found, under the service conditions of the tests, to average not over 100 hours total service, many plugs breaking down much sooner, while plugs constructed in accord with the present invention have demonstrated their ability to perform perfectly after more than 500 hours continuous service under the same conditions and in some cases after as much as 1000 hours continuous u service. Comparison of the ease of fouling has been made by removing the ignition wire from a plug and allowing the engine to run. Whereas ,the best available plugs of ordinary types al- Vmost invariably foul so badly as to require removal after a few revolutions, my improved type of plug haa/after being shorted out for over an hour, immediately i'lre, cleaned itself of any deposits of oil or carbon, and resumed operation without any impairment.

From the foregoing it will be seen thatxthe dimensions and limitations discussed are not denite or critical, but only serve to indicate that zone or range within which the-maximum advantages are secured from my improved construction. Similar advantages, but to a lesser degree, can be obtained at either side of the boundaries indicated, but it is believed that the various considerations governing design set out abovewhile individually are not fixed or definite, taken as a whole define with certainty the particular construction and advantages of my invention. Because changes indesign, size, and shape of parts may be made, it is desired that the foregoing description be considered as illustrative of rather than limitative upon the claims appended hereto. V

I claim:

1. In a spark plug for an internal combustion engine or the like, the combination of a body provided with an internal cavity; an electrode as-` sembly located ywithin said cavity and provided ywith. a sparking portion at its lower end; electrical insulating means interposed between the electrode assembly'and body to insulate onefrom the other; and an annular dielectric skirt associated with said insulating means and surrounding the lower end of the electrode'assembly but spaced therefrom, said skirt comprising a longitudinally laminated structure of dielectric material and havingk a thickness not over about .030 inch and a length-to-thickness ratio between 4 and 25.

2. In a spark -plug for an internal combustion' with a sparking portion at lits lower end; electrical insulating means interposed between the electrode assembly and body to insulate one from the other; an annulardielectric skirt associated with said insulating means and surrounding the lower end of the electrode assembly but spaced therefrom, said skirt comprising a longitudinally laminated structure of dielectric material; and a relatively thin metallic shell surrounding at least a portion of the skirt to reinforce and protect the latter'.

' 3. In a spark plug for an internal combustion engine or the like, the combination of a body; an electrode assembly located centrally of the bod-y; and a layer of electrical insulating material interposed between the electrode assembly and body'and for a portion of its length in direct engagement with the body and electrode assembly to insulate one from the other, said layer being formed of laminated dielectric material and extending beyond the body and into the combustion chamber of the engine to form an integral annular skirt portion annularlyjv spaced from both the body and the central electrode assembly. i l

4. In a spark plug for an internal combustion engine or the like, the combination of a body; a central electrode assembly surrounded by the body; electrical insulating means interposed between the body and electrode assembly, the insulating means comprising a laminated' mica layer surrounding the electrode assembly and extending beyond the limit of its engagement with the body to form an annular skirt portion that is spaced from both the body and the central elec'- trode assembly; and a thin metallic shell surrounding ohly a part of the skirt to reinforce andprotect vthe latter but leaving the lower end of the skirt exposed.

5. In a spark plug` for an internal combustiom engine or the like, the combination of a body; a central electrode assembly surrounded by the body; electrical insulating means interposed between the body and electrode assembly, the insulating means comprising a dielectric layer surrounding the electrode assembly; and an annular engine or the like, the combination ofa body; an

electrode assembly located centrally of the body; and a layer of electrical insulating material interposed between the electrode assembly and body and for a portion of itslength in engagement with the body and electrode assembly to insulate one from the other, said layer extending beyond the body and into the combustion chamber of the engine to form an annular skirt portion annularly spaced from the central electrode assembly, said skirt portion being of sufficiently small mass that it becomes suiciently hot at each explosion within the combustion chamber to burn 01T carbon deposits and becomes sufficiently cool between explosions not to ignite the new fuel charge in the chamber.

'7. In a spark plug for an internal combustion engine or the like, the combination of a body; a central electrode assembly surrounded by the body; electrical insulating means interposed between the body and electrode assembly, the insulating means comprising a dielectric layer surrounding the 'electrode assembly and extending beyond the limit of its engagement with the body to form an annular skirt portion that is spaced from both the body and the central electrode assembly; and a relatively thin metallic shell surrounding at least a portion of the skirt to reinforce and protect the latter, said skirt portion and metallic shell together being of suiiciently small mass that the skirt becomes sufliciently hot at each explosion within the combustion chamber to burn off carbon deposits and becomes sufficiently cool between explosionsv not to ignite the new fuel charge in the chamber.

8. In a spark plug for an internal combustion engine or the like, the combination of a body;

chamber of the engine to vform an annular skirt` portion annularly spaced from both the body and the central electrode assembly, said skirt portion being of suilciently small mass that it becomes suciently hot at each explosion within the combustion chamber to burn off carbon deposits and becomes sumciently cool between explosions not to ignite the new fuel charge in the chamber.

9. In a spark plug for an internal combustion engine or the like, the combination of a body; a central electrode assembly surrounded by the body; electrical insulating means interposed between the body and electrode assembly, the insulating means comprising a laminated mica layer surrounding the electrode assembly and extending beyond the limit of its engagement with the body to form an annular skirt portion that is spaced from both the body and the central electrode assembly; a thin metallic shell surrounding only a part of the skirt to reinforce and protect the latter but leaving the lower end of the skirt exposed; and an electrode mounted on the shell in a position to form with the central electrode assembly a spark gap.

10. In a spark plug for an internal combustion engine or the like, the combination of a body provided with a longitudinally extending cavity having a bottom section of smaller diameter than the upper section; an insulating liner within the upper section of said body cavity; a metallic shell lining the lower section of the cavity; a layer of insulation within the shell and extending both above and below the lower section of the cavity; a metal tube inside the insulation layer extending to near the top of the insulation layer and adapted to receive an electric conductor at its upper end; a plug carrying an electrode inside the lower end of the tube and extending below the tube; and a second electrode mounted on said shell and forming a spark gap with the firstmentioned electrode.

ALEXANDER L. ROBB. 

